In the manufacture of valves, in most cases it is desirable to provide movable seat elements that establish sealing engagement with a valve element and to incorporate some suitable means for imparting a particular preload force to the valve seat elements in order to establish proper sealing engagement between the seat elements and the valve element. This preload force is of particular advantage in providing effective sealing ability of the valve under low pressure conditions. Where movable valve seat elements are so employed, preload forces of sealing engagement may be developed mechanically, such as by through mechanical energy developed by seat springs. Alternatively, preload forces may be developed hydraulically, such as through utilization of forces developed by line pressure, in order to assist in the development of seating forces. Sealing preload forces may also be developed by elastomeric sealing rings that are maintained under mechanical compression. Obviously, seating systems may also be employed incorporating a combination of mechanical and hydraulic seat energization. The present invention is directed specifically to valve mechanisms incorporating mechanical seat springs that function to urge valve seats and seat assemblies into sealing engagement with the valve element.
One common valve design of the prior art incorporates helical compression springs that are retained within spring recesses defined within the valve body and impart a spring force to the valve seat. Another popular valve design incorporates flat or belleville type springs, such as evidenced by U.S. Pat. Nos. 3,091,428 of Magos; 3,883,112 of Milleville et al; and 4,066,240 of Atkinson et al. Belleville seat springs are typically manufactured in a pre-set, permanently deformed, dished or frusto-conical configuration and are deformed from this pre-set configuration during assembly of the valve mechanism, thus developing a preload force acting against the valve seat, urging the valve seat toward the valve element. In some cases, as evidenced by U.S. Pat. No. 3,834,666 of Keith, annular thrust developing springs may be utilized that are of flat configuration when unloaded.
One of the problems that is typically characteristic of belleville or flat type valve seat springs is the difficulty of such springs to compensate for manufacturing tolerances and maintain a preload force against the valve seat that is within acceptable design limits from the standpoint of sealing force and torque development. Flat or belleville springs are typically utilized under circumstances where internal movement of the valve mechanism is maintained at a minimum. Springs of this character develop and release stored mechanical energy with limited linear movement of the valve seat. Where valves are manufactured with relatively wide manufacturing tolerances, the variations due to such tolerances can result in sufficient seat movement as to be adverse to the development of optimum sealing ability and maintenance of operating torque within desired minimum operating levels. In other words, in order to ensure proper seating force and optimum operating torque when belleville springs are employed, it is typically desirable to manufacture the various valve parts with minimum tolerance variations. It is well known, however, that manufacturing costs increase exponentially as the range of manufacturing tolerance variations is minimized. It is desirable from a competitive standpoint to manufacture a valve mechanism where rather wide manufacturing tolerances are allowed, thus promoting low cost manufacture and insuring commercial feasibility of the valve product involved.
Another difficulty associated with the use of flat or belleville type seat springs in valves is the problem of hydrogen embrittlement and/or stress corrosion that may occur when such valve mechanisms are utilized under service conditions involving high concentrations of hydrogen sulfide. In the petroleum industry, petroleum products that are produced from many subsurface formations, known as sour gas or sour crude formations, contain high concentrations of hydrogen sulfide. When mechanical valve parts are composed of high carbon, rather hard metal spring materials and are maintained in stressed condition during service, the presence of hydrogen sulfide can result in rapid deterioration of these parts. It is desirable to provide seat springs for valves that are composed of materials that are not particularly susceptible to rapid deterioration in the presence of hydrogen sulfide.
Where valves utilizing conventional seat spring materials are manufactured with wide manufacturing tolerances, allowing considerable variation in the dimension of parts, it may be necessary to utilize different sized seat springs to compensate for variations in the manufacturing tolerances. It is desirable to provide flat or belleville type seat springs for valve mechanisms and to utilize a universal type belleville spring that will effectively compensate for variations in manufacturing tolerances and maintain optimum seating force and torque development.